While it’s possible to go down many levels in the navigation, it makes for a less ‘cleaner’ look. In the picture below, ‘TSW Overview’ is expandable, and ‘Scheduling Info’ shows the expansion.
Contrast that with the navigation below, which doesn’t have any submenus.
There may be times when sub-menus are needed, but try to limit them. Using the Sitemap feature which gives a list of pages, can help eliminate the need for subpages.